The invention relates to a SQUID system having an increased flux voltage transfer function.
The present invention refers to a superconducting quantum interference device, dc SQUID adapted for the very sensitive measurement of the magnetic flux and other physical quantities which may be converted into a magnetic flux (such as, for example, magnetic fields, magnetic field gradients, electrical current, and so forth).
The superconducting quantum interference devices, dc SQUIDs are at present the most sensitive detectors for determining a magnetic flux. Their function is based upon two physical phenomena, namely upon the quantizing of the magnetic flux within a superconducting loop and upon the Josephson Effect of two weakly coupled superconductors. A SQUID is a superconducting loop interrupted by at least one Josephson contact. There are known two basic kinds of SQUIDs [refer to A. Barone, G. Paterno: “Physics and Application of the Josephson Effect” A Wiley-Interscience Publication, John Wiley & Sons, New York (1982)], namely the rf SQUID and the dc SQUID. A superconducting loop which has only one Josephson contact is called rf SQUID. It is coupled to a radiofrequency tank circuit and in general is operated by an rf flux. The loop which is interrupted by at least two Josephson contacts is named dc SQUID, because it mostly is operated by a direct current (in this instance refer to FIG. 1a) flowing through the at least two contacts. The present invention refers to the use of a dc SQUID which is the sensitive element.
The critical current of the Josephson contact is a periodical function of the phase difference of the wave function of the charge carriers passing the contacts, whereby the phase difference occurs across the contact. The magnetic flux permeating the loop determines the phase difference, wherein a magnetic flux quantum Φ0=h/2=2.0710·10−15 Vs represents a characteristic magnitude. Therefrom the periodic dependency of the critical current results as a function of the flux with the period of a magnetic flux quantum Φ0 (refer to FIG. 1b). When the SQUID is operated with dc current the voltage drop across the Josephson contacts is also a periodic function of the magnetic flux permeating the loop (refer to FIG. 1c). Thus, the SQUID is a flux-voltage-transducer. The typical voltage rise of the presently available SQUIDs is about 50 μV and the maximal value of the transfer function is about 200 μV/Φ0. The typical SQUIDs only permit measurements of variations in the magnetic flux of far less than one flux quantum Φ0. The intrinsic SQUID noise can be of an order of 10−6 Φ0/Hz1/2 [T. Ryhänen, H. Seppä, R. llomoniemi and J. Knuutila: “SQUID Magnetometers for Low-Frequency Applications” J. Low Temp. Physics, 76, pp 287-386 (1989)].
In order to be able to use a SQUID sensor it is necessary to linearize the periodical flux-voltage characteristic. The respective standard method is to employ the SQUID as a zero detector. To this end, a flux variation which is to be measured will be compensated via a current flowing through a coil (feedback coil) which is inductively coupled to the SQUID. FIG. 2a shows an example for such a known solution according to the state of art. The Fig. schematically shows a SQUID in combination with a read-out electronics, the so-called feedback electronics or control loop. The feed-back electronics 5 picks up the voltage (V) from the SQUID 1, compares said voltage to a reference voltage (Vref) and amplifies the difference. When the feed-back switch (fb ON/OFF) is closed, a feed-back current (Ifb) flows through a feed-back coil 2, whereby the feed-back current compensates the flux changes effected by a signal, thus the entire flux is kept constant in the SQUID. This means that the SQUID takes a stable point upon the flux-voltage characteristic (one of the circles in FIG. 2b). Hence, due to the characteristic being periodical, there exist numerous of such stable operating points which are remote from one another by an integer number of magnetic flux quanta. A SQUID together with this shown feedback electronics is known as the so-called flux-locked-loop (FLL). The output voltage of the FLL (Vout) is directly proportional to the current (Ifb) which is required for compensating the signal flux.
Two basic kinds of FLL-electronics are known, these are the flux modulated electronics and the directly coupled electronics [J. Clarke, A. Braginski: “The SQUID Handbook” WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, (2006)].
The first mentioned employs the method of flux modulation to obtain an alternating voltage signal from a dc SQUID operated with a dc current. A transformer network provides for matching the impedances of the low-ohmic SQUID to the high-ohmic input of the first amplifier stage of the FLL-electronics. Then, after demodulation and integration, the signal will be passed as a current through the feedback coil. In such an electronics system the band width and the signal sequence rate (slew rate) are limited by the modulation frequency so that it cannot be used in many applications. The directly coupled electronics potentially functions also at higher frequencies and signal sequence rates. But the essential disadvantage of this electronics is the lack of matching the impedances of the SQUID to the amplifier input. Hence the intrinsic noise of the electronics as well as of the drift plays an important role in the noise limited resolution of the system. These problems occur above all at low frequencies, so that the application is restricted in some cases. Apart from the development of extremely low noise electronics, a SQUID transfer function increase is a possibility to evade the problem of an electronics noise. This is the reason why several approaches have been made to raise the flux-voltage-transfer function.
One possible way to raise the transfer function is the use of a multi-stage system wherein the SQUID signal of the first stage is amplified in a second one. The second stage can be a single SQUID (refer to FIG. 3a) or even an array of a number of series connected SQUIDs, wherein the SQUIDs are identical to one another as concerns their geometric formation and their electric parameters [refer to: R. P Welty, J. M. Martinis: “Two-stage integrated SQUID amplifier with series array output” IEEEE Trans. Applied Superconductivity, 3, pp. 2605-2608 (1993) and R. Cantor, L. P. Lee, A. Matlashov, V. Vinetskiy: “A Low-Noise, Two-Stage DC SQUID Amplifier with High Bandwidth and Dynamic Range”, IEEE Trans. Applied Supercond. 7, pp. 3033-3036 (1997)].
Thus, the obtainable raise of the flux-voltage-transfer function according of the prior art is exemplified in FIG. 3b. One disadvantage of the respective two-stage-system is also made visible in FIG. 3b. The resulting flux-voltage curve is very complicated due to the periodicities of the SQUID and in the SQUID-array and leads to a number of stable operation points of the FLL having respective different transfer functions and noise characteristics.
A further possibility to linearize the flux-voltage characteristic is to directly feed back the output signal of a multistage-SQUID system. This approach was made in K. D. Irwin, M. E. Hubert: “SQUID Operational Amplifier”, IEEE Trans. Applied Supercond. 11, 1265-1270 (2001). The idea of this approach is to feedback a portion of the operational current (IB2) of the last SQUID-array stage into the feedback coil of the SQUID of the first stage (refer to FIG. 4). Thus, the read-out electronics being at ambient temperature is reduced to a simple amplifier. However, the disadvantage of this approach remains the same. When the flux control loop is interrupted due to external interferences, a new not foreseeable operation point will set.
Recently, a new type of a superconducting multi-contact arrangement has been proposed, named Superconducting Quantum Interference Filter (SQIF) [EP 1 135 694 B1; DE 100 43 657 A1] for an entirely different application, namely, intended for a high precision absolute measurement of time-variable magnetic fields. This proposal substantially concerns an array out of a plurality of differently designed SQUIDs which are connected either in series or in parallel. A flux-current characteristic having only one extremum (maximum of the critical current or minimum of the voltage at a constant operating current) can be achieved by a special arrangement of the SQUID areas. Said extremum lies in the vicinity of the point at which the flux in each of the loops which form the SQIF is zero. This permits the absolute measurement of the magnetic flux and field, respectively, whereas the standard SQUID-systems only permit the measurement of the variations of the magnetic flux. A detailed description of the setup and function of the SQIF can be found in J. Oppenländer, Th. Träuble, Ch. Häussler, N. Schopohl: “Superconducting Multiloop Quantum Interferometer”, IEEE Trans. Applied Supercond. 11, pp. 1271-1274 (2001).